Electronic ballast for a discharge lamp

ABSTRACT

An electronic ballast for a discharge lamps has a dimming control for varying an input DC voltage and an input driving frequency in combination to achieve a lamp dimming in accordance with a dimmer demand. The ballast includes a dimming processor with a memory for storing a table that gives a predetermined relation between the dimming demand and a voltage command designating the input DC voltage as well as a frequency command designating the input driving frequency. A command processing unit is included in the processor to derive the voltage and frequency commands that matches with the dimmer demand so as to vary the light output of the lamp in accordance with the voltage command and the frequency command, respectively derived from the table.

TECHNICAL FIELD

[0001] The present invention relates to an electronic ballast for adischarge lamp, and more particularly to an electronic ballast enhancedwith a microcomputer technology for optimum dimming control.

BACKGROUND ART

[0002] Japanese Patent Publication No. 6-76979 (published on Mar. 18,1994) discloses an electronic ballast capable of dimming a dischargelamp. The ballast includes a DC voltage regulator providing a variableDC voltage and an inverter converting the DC voltage into a highfrequency AC power which is applied through a resonance circuit to thedischarge lamp. The DC voltage regulator is controlled by a voltagecommand to regulate or vary the DC voltage being applied to theinverter, while the inverter is controlled by a frequency command tovary the AC power being applied to the discharge lamp. The frequencycommand designates a frequency at which the inverter is driven tooscillate for generating the AC power. Also included in the ballast is adimming controller which, in response to an external dimming signal,provides the voltage command as well as the frequency command forregulating the DC voltage output from the DC voltage regulator and theAC power output from the inverter for dimming the lamp. The dimmingcontroller is configured to periodically change the frequency betweentwo different values in order to operate the lamp stably in a low lightoutput range. For this purpose, the ballast includes afrequency-switching unit which, in response to a monitored lamp current,instructs the dimming controller to change the frequency. Thus, thedimming control is made relying upon the lamp current being monitored.However, the dimming control may be insufficient for stably operatingthe lamp at different dimming levels due to a possible and transientfluctuation of the monitored lamp current. Thus, there is a demand forstably operating the lamp consistently at different dimming levels.

DISCLOSURE OF THE INVENTION

[0003] In view of the above insufficiency, the present invention hasbeen achieved to provide an electronic ballast for a discharge lampwhich is capable of operating the lamp stably at different dimminglevels. The ballast of the present invention includes a DC voltageregulator providing a DC voltage that varies with a voltage commandbeing fed to the DC voltage regulator, and an inverter converting the DCvoltage into a high frequency AC power which is applied through aresonance circuit to the discharge lamp. The high frequency AC power isregulated to vary with a frequency command which designates a frequencyat which the inverter is driven to oscillate. Included in the ballast isa dimming processor including a dimming unit that provides a dimmingsignal designating a varying dimming level for varying a light output ofthe discharge lamp in accordance with an external dimmer demand. Thedimming processor includes a command processing unit which, in responseto the dimming signal, generates the voltage command as well as thefrequency command. The frequency command is composed of a firstfrequency and a second frequency which is a function of the firstfrequency and gives a resulting lamp voltage higher than that obtainedat the first frequency. The command processing unit drives the inverterat the first and second frequencies alternately with respect to time tothereby regulate the high frequency AC power of the inverter.

[0004] The command processing unit is designed to include a memory forstoring a first table that determines a predetermined relation betweenthe dimmer signal and voltage command as well as the frequency command.It is from this memory that the command processing unit derives thevoltage command as well as the frequency command that matches with thedimmer signal so that the command processing unit operates the DCvoltage regulator and the inverter in accordance with the voltagecommand and the frequency command, respectively.

[0005] Thus, the first table in the memory can represent an optimumrelation between the dimming level and the voltage command as well asthe frequency command for the discharge lamp, thereby enabling thedimming control consistently and stably without being affected by apossible and transient variation in the lamp current or the likeparameters.

[0006] Preferably, the command processing unit provides a first timeframe and a second time frame in which the inverter operates at thefirst frequency and at the second frequency, respectively. The commandprocessing unit generates the second frequency which becomes lowertowards a middle of the second time frame than at the beginning and endof the second time frame. Thus, the second frequency can be given awaveform for increasing and decreasing the resulting AC voltage withinthe second time frame in an optimum manner for controlling the invertereffectively without posing undue stress to switching transistors formingthe inverter. Further, such optimum waveform can be given with theinclusion of the memory in the ballast.

[0007] It is preferred that the first table has at least one point ofinflection for a rate of voltage change of the voltage command and for arate of frequency change of the frequency command with respect to thevarying light output level. At least one point of inflection correspondsto a specific point with regard to the light output level, and defines alow light output range and a high light output range respectively belowand above the specific point. The first table determines the rate ofvoltage change which is different for the low and high light outputranges, and the rate of frequency change which is also different for thelow and high light output ranges. With this arrangement, the lamp can besuccessfully dimmed both in the low and high light output ranges wherethe lamp exhibits different lamp characteristics with regard to thevoltage and the frequency changes.

[0008] The first table may be configured to make the rate of voltagechange greater in one of the low and high light output ranges than inthe other range when the rate of frequency change is smaller in the oneof the low and high output ranges than in the other range.

[0009] Further, the first table may be configured to make at least oneof the rate of voltage change and the rate of frequency change differentfor the low and high light output ranges such that a rate of change inthe resulting light output of the lamp is smaller in the low lightoutput range than in the high light output range.

[0010] Still further, the first table can be designed to have at leasttwo points of inflection for the rate of voltage change and for the rateof the frequency change with respect to the varying light output level.The two points of inflection correspond to a first specific point and asecond specific point with regard to the light output level, and definea low light output range below the first specific point, an intermediatelight output range between the first and second specific points, and ahigh light output range above the second specific point. In this case,the first table is designed to make the rate of voltage change minimumin the intermediate light output range, and to make the rate offrequency change maximum in the intermediate light output range. Withthis control scheme, it is readily possible to dim or vary the lightoutput smoothly over a wide range in consideration of the lampcharacteristic.

[0011] Preferably, the memory includes, in addition to the first table,a second table which determines another predetermined relation betweenthe dimmer signal and the voltage command as well as the frequencycommand, different from the relation given by the first table. Inassociation with the first and second tables, a selector is included inthe command processing unit to derive the voltage command and thefrequency command from selective one of the first and second tables. Inaddition, the command processing unit includes a monitor which monitorsa rate of change in the dimming level intended by the dimming signal andwhich provides a first signal when the rate of change in the dimminglevel is smaller than a predetermined rate and otherwise provides asecond signal. The first signal actuating the selector to derive thevoltage command and the frequency command from the first table, whilethe second signal actuating the selector to derive the voltage commandand the frequency command from the second table. Thus, the dimmingcontrol of the lamp can be made differently depending upon whether thelamp is intended to vary its light output quickly or slowly, therebyenabling to vary the light output in a manner natural to the humanperception. For example, when the lamp is intended to increase its lightoutput quickly from minimum to maximum, one of the tables is selected togive a quick control not leaving a time lag in reaching the maximumlight output level.

[0012] In a preferred embodiment, the command processing unit includes avoltage signal generator that generates the voltage command, a firstfrequency signal generator that generates a first frequency commanddesignating the first frequency, a second frequency signal generatorthat generates a second frequency command designating the secondfrequency, and a selector providing selective one of the first andsecond frequency commands to the inverter. The voltage signal generatoris in the form of a pulse-width-modulator that gives a PWM signal ofwhich width defines the voltage command. The voltage signal generatorproduces a timing pulse in synchronize with the PWM signal and providesthe timing pulse to the second frequency signal generator. Each timeupon receiving the timing pulse, the second frequency signal generatorresponds to generate the second frequency command as well as apredetermined time frame. The selector is connected to receive the firstfrequency command, the second frequency command, and the time frame sothat it passes the second frequency command to the inverter only duringthe time frame and otherwise pass the first frequency command to beissued to the inverter. With this arrangement, the voltage signalgenerator is best utilized to determine the time frame for passing thesecond frequency command without relying upon an additional circuitcomponent specifically designed to determine the timing of changing thefirst and second frequency commands.

[0013] The command processing unit may include one or more smoothingcircuits which smooth out at least one of the first frequency command,the dimming signal, the voltage command, and the frequency command foravoiding a possible abrupt error that may occur in the control of thedischarge lamp.

[0014] The smoothing circuits may be configured to have adjustable timeconstants which determine individual response times by which the voltagecommand and the frequency command are delayed in driving the DC voltageregulator and the inverter, respectively. A dimming direction monitor isincluded in the command processing unit to provide an upward signal whenthe dimming signal indicates an increase in the light output level andprovide a downward signal when the dimming signal indicates a decreasein the light output level. The dimming direction monitor adjusts thetime constants of the smoothing circuits in order to make the responsetime of the voltage command longer than that of the frequency command inresponse to the upward signal, and to make the response time of thefrequency command longer than that of the voltage command in response tothe downward signal. Thus, it is possible to vary the light output ofthe lamp smoothly and naturally with different delays, i.e., in anoptimum fashion natural to the human perception depending upon thedirection in which the lamp is intended to vary the light output.

[0015] The frequency command is realized by a digital signal to beconverted by a D/A converter into an analog signal which is fed to avoltage controlled oscillator. The voltage controlled oscillator isprovided in the ballast to receive the analog signal and drive theinverter at the frequency designated by the frequency command. The D/Aconverter is preferably defined by a ladder resistor network or weightedresistor network.

[0016] These and still other objects and advantageous features of thepresent invention will become more apparent from the following detaileddescription of the embodiments when taken in conjunction with theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a block circuit diagram of an electronic ballast for adischarge lamp in accordance with a first embodiment of the presentinvention;

[0018]FIG. 2 is a waveform chart illustrating the operation of theballast;

[0019]FIG. 3 is a block diagram illustrating a portion of the ballast;

[0020]FIGS. 4A and 4B are graphs respectively illustrating a varyinglight output of the lamp as a function of a dimming signal;

[0021]FIGS. 5A and 5B are graphs illustrating a voltage index Vx and afrequency index Fx varying in accordance with the dimming signal (Dim);

[0022]FIGS. 6A and 6B are graphs illustrating a dimming control schemestored in a first table with regard to the voltage index Vx and thefrequency index Fx that vary in different manners in different lightoutput ranges, respectively;

[0023]FIGS. 7A and 7B are graphs illustrating a dimming control schemestored in a second table with regard to the voltage index Vx and thefrequency index Fx that vary in different manners in different lightoutput ranges, respectively,

[0024]FIGS. 8A and 8B are graphs illustrating another dimming controlavailable in the ballast;

[0025]FIG. 9 is a graph illustrating a simplified dimming controlavailable in the ballast with regard to a relation between the lightoutput (φ), the voltage index Vx, and the frequency index Fx;

[0026]FIG. 10 is a block diagram of a portion of the ballast inaccordance with a modification of the above embodiment;

[0027]FIG. 11 is a waveform chart illustrating the dimming control in adirection of increasing the light output (φ);

[0028]FIG. 12 is a waveform chart illustrating the dimming control in adirection of decreasing the light output (φ);

[0029]FIG. 13 is a graph illustrating a relation between the lightoutput (φ) and a time constant (T) of a particular smoothing circuitincluded in the ballast;

[0030]FIG. 14 is a block circuit diagram of an electronic ballast for adischarge lamp in accordance with a second embodiment of the presentinvention;

[0031]FIG. 15 is a waveform chart illustrating the operation of theballast; and

[0032]FIGS. 16A and 16B are graphs illustrating a correlation betweenthe voltage index Vx and the frequency index Fx that may be utilized inthe ballast of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0033] Referring now to FIG. 1, there is shown an electronic ballast fora discharge lamp in accordance with a first embodiment of the presentinvention. The ballast is designed for dimming the lamp, i.e., adjustingthe light output of the lamp, and includes a rectifier 10 providing arectified DC voltage from an AC voltage source, a DC voltage regulator20 providing a regulated DC voltage, and an inverter 30 powered by theoutput of the DC voltage regulator 20 to supply a high frequency ACpower to the discharge lamp 40. The inverter 30 includes a resonantcircuit through which the high frequency AC power is supplied to thedischarge lamp 40.

[0034] The DC voltage regulator 20 is in the form of a conventionalchopper having an inductor and a switching element which is driven toturn on and off repetitively to store an energy in the inductor whilethe switching element is on and release the energy, i.e., the DC voltagewhile the switching element is off, thereby regulating the output DCvoltage. A chopper controller 21 is included to control the DC voltageregulator 20 in accordance with a voltage command Vcmd supplied from adimming processor 100. The voltage command Vcmd is a digital signalwhich is smoothed through the low-pass filter 22 into a voltage signalVcv input to the chopper controller 21 which drives the DC voltageregulator to provide the DC voltage determined by the pulse-width of thevoltage signal Vcv. In addition, the chopper controller 21 monitors theoutput DC voltage in order to keep the output DC voltage in a feedbackmanner at a level intended by the voltage signal Vcv.

[0035] The inverter 30 is of a conventional design having switchingtransistors which are driven to turn on and off at a high frequency toconvert the output DC voltage into the high frequency AC power beingsupplied through the resonant circuit to operate the discharge lamp 40.The inverter 30 is controlled by a voltage controlled oscillator 31which gives, in accordance with a frequency command Fcmd from thedimming processor 100, a varying frequency signal Sf at which theswitching transistors are turned on and off. The frequency command Fcmdis a digital signal which is converted through a D/A converter 32 into acorresponding voltage signal Vcf input to the voltage controlledoscillator 31. The inverter 30 is designed to have a driving frequencywhich is variable within a range not lower than a resonant frequency ƒ₀of the resonant circuit in order to reduce the lamp voltage as thefrequency increases.

[0036] The dimming processor 100 is realized by a micro-processor unit(MPU) which processes an external dimmer demand Ddmd from a dimmer 90 togenerate the voltage command Vcmd and the frequency command Fcmd fordimming, i.e., adjusting the light output of the discharge lamp 40 asintended by the external dimmer demand Ddmd. The dimming processor 100includes a dimming unit 190 that converts the external dimmer demandDdmd of an analog signal into a corresponding digital dimming signal,and a command processing unit 110 that generates the voltage commandVcmd and the frequency command Fcmd based upon the dimming signal fromthe dimming unit 190. The command processing unit 110 includes a commandcreator 120 that provides a voltage index Vx and a frequency index Fx, aPWM signal generator 130 that generates the voltage command Vcmd incorrespondence to the voltage index Vx. The-voltage command Vcmd is apulse modulated signal of which pulse-width is proportional to the lightoutput of the discharge lamp intended by the dimming signal.

[0037] The terms “voltage command Vcmd” and “voltage index Vx” areintroduced in the description for the purpose of explaining theoperation of the ballast in exact coincidence with the illustratedcircuit configuration. However, as will be clear from the followingdescription, the term “voltage command Vcmd” has a direct relation tothe term “voltage index Vx”, and therefore these terms have equalweights in defining the scope of the present invention. Only for thesake of conciseness, the claims and some parts of the description recite“the voltage command” as representative of “the voltage index”. Thisapplies also to the relation between the terms “frequency command Fcmd”and “frequency index Fx”.

[0038] Also included in the command processing unit 110 are a firstfrequency signal generator 140 that generates a first frequency commandDf1 in correspondence to the frequency index Fx, and a second frequencysignal generator 150 that generates a second frequency command Df2 alsoin correspondence to the frequency index Fx. The first frequency commandDf1 designates a first frequency for driving the inverter, while thesecond frequency command Df2 designates a second frequency which is afunction of the first frequency to give a resulting lamp voltage higherthan that obtained at the first frequency.

[0039] The first and second frequency commands Df1 and Df2 arealternately supplied through a switch 170 and an output port 180 as thefrequency command Fcmd to the D/A converter 32. For this purpose, thecommand processing unit 110 further includes a timing pulse generator160 that determines a timing of changing over the first and secondfrequency commands, as shown in FIG. 2, in order to vary the drivingfrequency of the inverter 30 cyclically between those designated by thefirst and second frequency commands at the intended light output of thedischarge lamp. That is, the timing pulse generator 160 generates atiming pulse Tp for a predetermined duration regularly, as shown in FIG.2, so that the switch 170 acts to pass the first frequency command Df1only in the absence of the timing pulse Tp and to pass the secondfrequency command Df2 only in the presence of the timing pulse Tp. Inother words, the timing pulse generator 160 defines a first time frameTF1 in which the inverter 30 is driven to oscillate in response to thefirst frequency command Df1 and a second time frame TF2 in which theinverter 30 is driven to oscillate in response to the second frequencycommand Df2. Thus, the inverter 30 is driven to oscillate basically attwo alternate frequencies for cyclically varying the lamp voltage Vla inorder to keep the lamp operated without causing undesired extinctioneven at the low light output of the discharge lamp. In detail, thesecond frequency command Df2 designates a varying frequency whichbecomes lower towards a middle of the second time frame TF2 than at thebegging and the end of the second time frame TF2 in such a pattern asnot to cause undue stress applied to the switching elements of theinverter 30.

[0040] As shown in FIG. 3, the command creator 120 has a memory whichstores a first table 111 and a second table 112 each determining apredetermined relation between the dimming signal and the voltagecommand Vcmd as well as the frequency command Fcmd for providing thevoltage index Vx to the PWM signal generator 130 and the frequency indexFx to the first and second frequency signal generators 140 and 150. Thefirst and second tables 111 and 112 are provided to be reliedselectively upon in accordance with how fast the light output isintended to vary. When the light output is intended to vary moderately,the first table 111 is relied upon to give the relation, as shown inFIG. 4A, where the resulting light output follows the dimming ratio Dimwith some delay in response in order to vary the light output natural tothe human eyes. While on the other hand, when the light output isintended to vary quickly, the second table 112 is relied upon to givethe relation, as shown in FIG. 4B, where the resulting light outputfollows the dimming ratio Dim straight in order to vary the light outputquickly without causing a response delay in the human perception. Forthis purpose, the command creator 120 includes a differential unit 116which calculates a differentiated value of the dimming signal from thedimming unit 190 and causes a selector 114 to derive the voltage indexVx and the frequency index Fx from the first table 111 in correspondenceto the dimming ratio Dim when the differentiated value is lower than apredetermined level, and otherwise derives the voltage index Vx and thefrequency index Fx from the second table 112. The selection of the firsttable 111 and the second table 112 may be made based upon anotherparameter indicative of a rate of the changing the light output of thelamp. It is noted here that the dimming ratio Dim is utilized in thedescription to denote an intended light output that increases with theincreased dimming ratio.

[0041] Further, the command creator 120 includes a smoothing circuit 113that smoothes out the dimming signal to remove any unintended abruptfluctuation or error noise from the dimming signal being fed from thedimming unit 190 to the first and second tables 111 and 112. Likesmoothing circuits 117 and 118 are provided for smoothing out thevoltage index Vx and the frequency index Fx being fed to the PWM signalgenerator 130 and the first and second frequency signal generators 140and 150, respectively. Additional smoothing circuits 117 and 118 areprovided to smooth out the voltage index Vx and the frequency index Fx,respectively

[0042] The first table 111 defines a characteristic curve of the voltageindex Vx, i.e. the voltage command Vcmd that increases generally in aquadric manner as the dimming ratio Dim increases, i.e., the dimmingsignal gives the increasing light output, as shown in FIG. 5A, and acharacteristic curve of the frequency index Fx, i.e., the frequencycommand Fcmd that decreases generally in a quadric fashion as thedimming ratio Dim increases, as shown in FIG. 5B. These curves arecombined to represent the relation of FIG. 4A. In detail, as shown inFIGS. 6A and 6B, the first table 111 defines the characteristic curve ofthe voltage index Vx, that has two inflection points for a rate ofvoltage change of the voltage index Vx as well as for a rate offrequency change of the frequency index Fx. These two inflection pointsare set to correspond respectively to first and second specific pointswith regard to the dimming ratio Dim, and define a low light outputrange L below the first point, an intermediate light output range Mbetween the first and second specific points, and a high light outputrange H above the second specific point. The first table 111 gives therate of voltage change ΔVx which is minimum in the intermediate lightoutput range M, while giving the rate of frequency change ΔFx which ismaximum in the intermediate light output range M. Thus, it is easy tovary the light output of the discharge lamp optimally in the differentlight output ranges for achieving consistent dimming control over a widerange of the light output. On the other hand, the second table 112gives, as shown in FIGS. 7A and 7B, the rate of voltage change ΔVx whichis maximum in the intermediate light output range M, while giving therate of frequency change ΔFx which is minimum in the intermediate lightoutput range M, making it easy to achieve a consistent dimming controlover the wide range also with the second table 112.

[0043] The first table 111 or the second table 112 may be designed todefine the characteristic curves of the voltage index Vx as well as thefrequency index Fx both having three inflection points, as shown inFIGS. 8A and 8B, for giving the rates of voltage and frequency changesthat differ from regions to regions over substantially the entire rangeof the light output of the discharge lamp from the minimum to themaximum of the dimming ratio Dim.

[0044] Further, the first table 111 or the second table 112 may besimplified to define the characteristic curves of the voltage index Vxas well as the frequency index Fx each having only one inflection pointthat corresponds to a middle level γ of the light output, as shown inFIG. 9. In this instance, the voltage index Vx, i.e., the voltagecommand Vcmd is kept constant within a low light output region betweenthe minimum and the middle level γ, and increases with the dimming ratioDim increasing past the middle level γ to the maximum, while thefrequency command Fx decreases with the dimming ratio Dim decreasingfrom the minimum to the middle level γ, and is kept constant within ahigh output region above the middle level γ. It is noted in thisconnection that the first and second tables are configured todifferentiate at least one of the rate of the voltage index, the rate ofthe frequency index, and the middle level γ. As shown in FIG. 10, thecommand creator 120 having thus configured first and second tables 111and 112 has a direction monitor 119 that processes the dimming signalfrom the dimming unit 190 to obtain the direction of varying the lightoutput for differentiate the dimming control depending upon themonitored direction, realizing to vary the light output in consistentwith the human eye's perception. The direction monitor 119 provides anupward signal and a downward signal when the dimming ratio Dim increasesand decreases, respectively. Depending upon the monitored direction, oneof the smoothing circuit 117 for the voltage index Vx and the smoothingcircuit 118 for the frequency index Fx is controlled to delay inproviding the corresponding one of the indexes from the other. For thispurpose, the smoothing circuits 117 and 118 have adjustable timeconstants that determine individual response times by which the voltageindex Vx and the frequency index Fx are delayed in driving the DCvoltage regulator 20 and the inverter 30.

[0045] For example, as shown in FIG. 11, when the dimming ratio Dim isintended to increase from minimum to maximum at time t0, the smoothingcircuit 117 is controlled by the upward signal to increase the timeconstant by a greater extent than the smoothing circuit 118, in order todelay in increasing the voltage index Vx than the frequency index FXthat starts decreasing at time t0, and to start increasing the voltageindex at time t1. When, on the other hand, the dimming ratio Dim isintended to decrease from maximum to minimum at time t0, the smoothingcircuit 118 is controlled by the downward signal to increase the timeconstant by a greater extent than the smoothing circuit 117, as shown inFIG. 12. Consequently, the frequency index Fx is delayed in increasingwhile the voltage index Vx starts decreasing immediately at time t0, andthe frequency index Fx starts increasing only after time t1. With thisresult, the resulting light output responds to vary with some delay inconsistent with and natural to the human eye's perception.

[0046] Also for making the consistent dimming control natural to thehuman eyes, the smoothing circuit 113 just behind of the dimming unit190 is configured to decrease the time constant T with the increasingdimming ratio, as shown in FIG. 13, to provide the modified dimmingsignal Dx in order to delay providing the voltage index Vx and thefrequency index Fx by a greater extent as the dimming ratio Dimdecreases, i.e., at the time of varying the light output in the lowerlight output range than at the high light output range.

[0047] Although the present embodiment is explained to selectively usethe first and second tables, it is equally possible to rely on a singletable corresponding to either of the first and second table, or even toprovide or more like tables for selecting one of the tables inaccordance with the intended rate or direction of varying the lightoutput of the discharge lamp.

[0048] Turning back to FIG. 1, the D/A converter 32, which converts thefrequency command Fcmd into the analog signal Vcf, is made of a ladderresistor network or weighted resistor network. Instead of using thecombination of the voltage controlled oscillator 31 and the D/Aconverter 32, it is equally possible to provide a suitable oscillatorthat determines the driving frequency directly from the frequencycommand Fcmd to drive the inverter at thus determined driving frequency.

[0049] Referring to FIGS. 14 and 15, there is shown an electronicballast in accordance with a second embodiment of the present inventionwhich is identical to the first embodiment except that the PWM signalgenerator 130 is best utilized to give the timing pulse Tp for changingover the first and second frequency commands Df1 and Df2, eliminatingthe necessity of providing the separate timing pulse generator 160 asutilized in the first embodiment. The other structures and operationsare identical, therefore no duplicate explanation are made here.

[0050] As shown in FIG. 15, each time the PWM signal generator 130generates the voltage command Vcmd, it provides the timing pulse Tp thatis fed to the second frequency signal generator 150 which responds toissue a switching pulse Ts having a predetermined pulse width, inaddition to the second frequency command Df2. The switching pulse Tsactivates the switch 170 to pass the second frequency command Df2 to theoutput port 180 within the pulse width of the switching pulse Ts,thereby driving the inverter 30 at the frequency designated by thesecond frequency command Df2. In the absence of the switching pulse Ts,the switch 170 is caused to pass the first frequency command Df1 fromthe first frequency signal generator 140 to the output port 180 in theabsence of the switching pulse Ts. Thus, the second frequency signalgenerator 150 determines, in response to the timing pulse Tp from thePWM signal generator 130, the first and second time frames TF1 and TF2which repeat alternately for driving the inverter 30 cyclically at thedifferent frequencies defined respectively by the first and secondfrequency commands Df1 and Df2, as shown in FIG. 15.

[0051] In the above illustrated embodiments, it is explained that thecommand creator 120 provides the voltage index Vx and the frequencyindex Fx separately based upon the dimming signal from the dimming unit190, i.e., each of the tables stores the voltage index Vx and thefrequency index Fx defined separately for different dimming ratios.However, it is equally possible to correlate the voltage index Vx withthe frequency index Fx, as shown in FIGS. 16A and 16B. That is, thevoltage index Vx is defined as a function of the dimming ratio Dim, andthe frequency index Fx is defined as a function of the voltage index Vx.Thus, the structure of the table can be simplified to store only thevoltage index Vx, with an addition of a simple multiplier in the commandcreator 120 that multiplies the voltage index Vx by a proper value togive the correlated frequency index Fx.

[0052] It is noted that the present invention should not be limited tothe features disclosed as specific to the individual embodiments andmodifications, and should encompass any combination of the individualfeatures. This application is based upon and claims the priority ofJapanese Patent Application No. 2002-154804, filed in Japan on May 28,2002, the entire contents of which are expressly incorporated byreference herein.

1. An electronic ballast for a discharge lamp comprising: a DC voltageregulator providing a DC voltage that varies with a voltage commandbeing fed to said DC voltage regulator, an inverter converting said DCvoltage into a high frequency AC power which is applied through aresonance circuit to said discharge lamp, said high frequency AC powerbeing regulated to vary with a frequency command which designates afrequency at which said inverter is driven to oscillate; a dimmingprocessor including a dimming unit that provides a dimming signaldesignating a varying dimming level for varying a light output of saiddischarge lamp in accordance with an external dimmer demand, whereinsaid dimming processor includes a command processing unit which, inresponse to the dimming signal, generates said voltage command and saidfrequency command, said frequency command designating a first frequencyand a second frequency which is a function of said first frequency andgives a resulting lamp voltage higher than that obtained at said firstfrequency, said command processing unit driving said inverter at saidfirst and second frequencies alternately with respect to time to therebyregulate said high frequency AC power of said inverter, said commandprocessing unit including a memory for storing a first table thatdetermines a predetermined relation between the dimming signal andvoltage command as well as said frequency command, said commandprocessing unit deriving from said memory said voltage command as wellas said frequency command that matches with the dimming signal, andoperating said DC voltage regulator and said inverter in accordance withsaid voltage command and said frequency command, respectively.
 2. Theelectronic ballast as set forth in claim 1, wherein said commandprocessing unit provides a first time frame in which said inverteroperates at said first frequency and a second time frame in which saidinverter operates at said second frequency, said command processing unitgenerating said second frequency which becomes lower towards a middle ofsaid second time frame than at the beginning and end of said second timeframe.
 3. The electronic ballast as set forth in claim 1, wherein saidfirst table has at least one point of inflection for a rate of voltagechange of said voltage command and for a rate of frequency change ofsaid frequency command with respect to the varying light output level,said at least one point of inflection corresponding to a specific pointwith regard to the light output level, and defining a low light outputrange and a high light output range respectively below and above saidspecific point, and said first table determining said rate of voltagechange which is different for said low and high light output ranges, anddetermining said rate of frequency change which is different for saidlow and high light output ranges.
 4. The electronic ballast as set forthin claim 3, wherein said first table determines that the rate of voltagechange is greater in one of said low and high light output ranges thanin the other when the rate of frequency change is smaller in said one ofsaid low and high output ranges than in the other.
 5. The electronicballast as set forth in claim 1, wherein said first table has at leastone point of inflection for at least one of a rate of voltage change ofsaid voltage command and of a rate of frequency change of said frequencycommand with respect to the varying light output level, said at leastone point of inflection corresponding to a specific point with regard tothe light output level, and defining a low light output range and a highlight output range respectively below and above said specific point, andsaid first table determining at least one of said rate of voltage changeand said rate of frequency change is different for said low and highlight output ranges such that a rate of change in the resulting lightoutput of said discharge lamp is smaller in said low light output rangethan in said high light output range.
 6. The electronic ballast as setforth in claim 1, wherein said first table has at least two points ofinflection for a rate of voltage change of said voltage command and fora rate of frequency change of said frequency command with respect to thevarying light output level, said two points of inflection correspondingto a first specific point and a second specific point with regard to thelight output level, and defining a low light output range below thefirst specific point, an intermediate light output range between thefirst and second specific points, and a high light output range abovesaid second specific point, and said first table determining said rateof voltage change which is minimum in said intermediate light outputrange, and said rate of frequency change which is maximum in saidintermediate light output range.
 7. The electronic ballast as set forthin claim 1, wherein said memory includes a second table in addition tosaid first table, said second table determining another predeterminedrelation between the dimming signal and said voltage command as well assaid frequency command, which is different from said relation given bysaid first table, said command processing unit including a selector forderiving said voltage command and said frequency command from selectiveone of said first and second tables, said command processing unit alsoincluding a monitor for monitoring a rate of change in the dimming levelintended by the dimming signal and provide a first signal when said rateof change in the dimming level is smaller than a predetermined rate andotherwise provides a second signal, said first signal actuating saidselector to derive said voltage command and said frequency command fromsaid first table, and said second signal actuating said selector toderive said voltage command and said frequency command from said secondtable.
 8. The electronic ballast as set forth in claim 1, wherein saidcommand processing unit includes: a voltage signal generator whichgenerates said voltage command, a first frequency signal generator whichgenerates a first frequency command designating said first frequency, asecond frequency signal generator which generates a second frequencycommand designating said second frequency command, a selector forproviding selective one of said first and second frequency commands tosaid inverter said voltage signal generator being apulse-width-modulator that gives a PWM signal of which width definessaid voltage command, said voltage signal generator producing a timingpulse in synchronize with said PWM signal and providing said timingpulse to said second frequency signal generator, said second frequencysignal generator generating said second frequency command and apredetermined time frame each time it receives said timing pulse, andsaid selector being connected to receive said first frequency command,said second frequency command, and said time frame so that it passessaid second frequency command to said inverter only during said timeframe and otherwise pass said first frequency command to be issued tosaid inverter.
 9. The electronic ballast as set forth in claim 1,wherein said command processing unit includes a smoothing circuit forsmoothing out said first frequency command.
 10. The electronic ballastas set forth in claim 1, wherein said command processing unit includes afirst smoothing circuit smoothing out said dimming signal, a secondsmoothing circuit for smoothing out said voltage command, and a thirdsmoothing circuit for smoothing out said frequency command.
 11. Theelectronic ballast as set forth in claim 10, wherein said second andthird smoothing circuits have adjustable time constants which determineindividual response times by which said voltage command and saidfrequency command are delayed in driving said DC voltage regulator andsaid inverter, respectively, said command processing unit including adimming direction monitor which monitors said dimming signal to providean upward signal when said dimming signal indicates an increase in thelight output level and provide a downward signal when said dimmingsignal indicates a decrease in the light output level, said dimmingdirection monitor adjusting said time constants of said second and thirdsmoothing circuits in order to make the response time of said voltagecommand longer than that of said frequency command in response to saidupward signal, and to make the response time of said frequency commandlonger than that of said voltage command in response to said downwardsignal.
 12. The electronic ballast as set forth in claim 1, wherein saidfrequency command is a digital signal which is converted by a D/Aconverter into an analog signal which is fed to a voltage controlledoscillator, said voltage controller receiving said analog signal todrive said inverter at the frequency designated by said frequencycommand, and said D/A converter being defined by a ladder resistornetwork or weighted resistor network.